Process for the purification of polymer solutions

ABSTRACT

Polymer solutions are purified with a detergent solution in pulsating columns with perforated bases, in which a proportion by volume of the polymer solution to the detergent solution of &lt; OR = 0.1 is maintained in the extraction column and the detergent solution is used as the continuous phase.

United States Patent Morgenstern et al.

[ 51 May 30, 1972 [54] PROCESS FOR THE PURIFICATION OF POLYMER SOLUTIONS[72] Inventors: Karl Morgenstem, Krefeld; Hermann Schnell,Krefeld-Uerdingen; Ludwig Bottenbruch, Krefeld-Boekum; Otto Court,Neuss; Hans-Helmut Schwarz, Krefeld- Bochum; Hugo Vernaleken, Krefeld,all of Germany [73] Assignee: Farbenfabriken Bayer Aktiengesellschaft,

Leverkusen, Germany [22] Filed: Feb. 24, 1970 211 App]. No.1 13,853

[30] Foreign Application Priority Data Mar. 5, 1969 Germany ..P 19 11179.6

[521 U.S. Cl. ..260/47 XA, 260/96 R Primary Examiner-Samuel H. BiechAttorneyBurgess, Dinklage & Sprung [57] ABSTRACT Polymer solutions arepurified with a detergent solution in pulsating columns with perforatedbases, in which a proportion by volume of the polymer solution to thedetergent solution of $0.] is maintained in the extraction column andthe detergent solution is used as the continuous phase.

5 Claims, No Drawings PROCESS FOR THE PURIFICATION OF POLYMER SOLUTIONSThe present invention relates to a process for the purification ofpolymer solutions by washing them in a pulsating column which has aperforated base.

Polycondensates which are obtained by phase boundary surface reactions,e.g., polycarbonate obtained by the action of phosgene on bisphenol inthe presence of aqueous alkali and organic solvents, often containunreacted starting materials and salts as impurities derived from themanufacturing process. These substances interfere with subsequentprocesses and they impair the quality of the end product unless they arecompletely removed.

It is therefore necessary to free the reaction solution from theimpurities by evaporation of the solvent before the polymer is isolated.This removal of impurities is achieved by washing the polymer solution.Suitable devices for carrying out the washing process are, for example,combinations of high speed mixers and high speed centrifuges. Thesedevices are expensive to buy and to maintain and, moreover, it is ofiennecessary to arrange several such devices in a row in order to wash outthe impurities completely. Extraction columns, which generally operatemore economically and have distinct advantages with regard to theirinitial running and maintenance costs when compared with the costs whenmixer centrifuges are used, have not hitherto been described for use inthe extraction of polymer solutions. Their use is ofien precluded by thetendency of viscous polymer solutions to emulsify with the detergent inthe columns. This applies particularly to so-called rotating andpulsating columns. Columns having packing materials, in which thepacking materials have to be packed fairly tightly in order to obtain aconcentrated and efficient exchange of material, retain the viscouspolymer solutions between the packing materials so that no droplets areformed and, therefore, no large surface areas are formed and no exchangeof material takes place.

Surprisingly, it has now been found that polymer solutions can be washedin pulsating columns having perforated bases if the proportion by volumeof polymer solution to detergent solution in the extraction column ismaintained at 0.1 and the detergent solution is used as the continuousphase.

In this case, no blockage occurs due to an accumulation of polymersolution on the perforated base and the formation of an emulsion isprevented. High washing efficiency is achieved using polymer solutionsof viscosities lying between and 100 cP.

The following variable conditions are of great importance for theoptimum and complete removal of impurities and salts from the polymersolution by washing:

The loading capacity (m/h) may be varied between the limits of 10 and 20m /h of total solution per m of cross-section of the column.

The distance between the individual perforated bases should be between 5and cm.

Considering now the free surfaces of the perforated bases and thediameter of the perforations: These two parameters are of greatimportance in influencing the efficiency and feasibility of using thecolumns having perforated bases for the extraction of polymer solutions.In order to prevent an accumulation of the polymer solutions on theperforated bases, a free surface of more than 15 percent especiallypercent and up to percent is required, and the diameter of theperforations should be between 1.5 and 4.0 mm especially between 1.5 and2.5 mm in order to ensure a fresh formation of surfaces without thedroplets passing through the perforations without touching them. Theshape of the perforations is of no particular importance, but circularor elliptical perforations were found to be the most suitable.

Considering now the lifting height and pulsating frequency: A largelifting height increases the residence time and hence the proportion ofpolymer solution in the continuous phase, but it has the same efiect asan elevated pulsation frequency of breaking up the polymer droplets onthe perforated bases. If

the pulsation frequency is too low, the polymer solution remains on theperforated bases without passing through them. Suitable lifting heightsare in the region of between 2 and 5 mm, and the pulsation frequency maybe adjusted to values of between 50 and 300 strokes per minute. Optimumresults are obtained with stroke frequencies of between and 150 strokesper minute.

Considering now the difference in density between the polymer solutionand the detergent solution, which is of great importance for the rate ofsinking and the total residence time of the polymer solution in thedetergent solution: To maintain a sufficiently high sinking velocity,differences in density of more than 0.1 g/em are generally sufficient,but polymer solutions which differ in density from the detergentsolution by only 0.5 g/cm can also be extracted if sufficiently largelifting heights and low stroke frequencies are employed.

Another important factor for the removal of the polymer solution bywashing is the ratio by volume of the throughput of polymer solution tothe detergent solution; this ratio may be varied within the limits offrom 10:1 to 1:1.

Columns designed with perforated bases and equipped with pulsatingdevices are suitable for carrying out the process of the invention.Solvents which are not miscible with the polymer solution and which donot react with it are suitable for use as detergents. Dilute sodiumhydroxide solution is preferably used for extracting acid substances,dilute phosphoric acid is used for extracting basic impurities andconductivity water is used for the extraction of salt.

EXAMPLES A solution of polycarbonate in chlorobenzene/methylene chloridehaving a density of 1.21 g/cm and a viscosity of 40 cP was used for theExamples. The polycarbonate solution contained from 0.2 to 1.0 percentof water with a chlorine ion concentration of 5 g of Cl/ 1 00 g ofwater.

COMPARISON EXAMPLE 1 The apparatus used was a column containing packingmaterials and having an effective height of 2,500 mm and a diameter ofmm. The following were used as packing material:

a. Saddle pieces, 5 mm, 10 mm b. Raschig rings, 5 mm, 10 mm Whenattempts were made to extract the polycarbonate solution in counterflowwith water, it was found that the polycarbonate solution remained in theupper zones of the layer of packing material. Extraction was notpossible.

COMPARISON EXAMPLE 2 In a rotating column which had an efiective heightof 2,700 mm and a diameter of 35 mm and 50 whirling zones and 50 contactzones filled with 5 mm Rashig rings of V2A wire mesh, polycarbonatesolution was extracted in counterflow with water at speeds of rotationof between 100 and 450 revs/min. Even when the rates of throughflow wereonly 4 liters of polycarbonate solution per hour and 1 liter of waterper hour, no satisfactory extraction was found to take place.

In the conductivity test in which one part of the extractedpolycarbonate'solution and one part of conductivity water having aconductivity of 0.1 10 S/cm are thoroughly mixed for 5 minutes, theconductivity of the water was found to be greater than 5 10 S/cm. Thepolycarbonate still contained ppm of Cl after the extraction test. Theefficiency of the extraction also could not be improved by increasingthe rate of stirring to about 1,000 revs/min. The polycarbonate solutionemulsified at these high stirring speeds.

EXAMPLE 3 In a pulsating column with a perforated base which had aneffective height of 2,200 mm and a diameter of 35 mm and contained 22perforated bases having a perforation diameter of 2.0 mm and a freesurface of 20 percent, l4 liters of polycarbonate solution per hour wereextracted in counterflow with 2 liters of water per hour at a pulsationfrequency of 140 strokes per minute and a lifting height of 5 mm.

The polycarbonate solution could be washed free from salt (less than 2ppm Cl). A conductivity of 0.1 S/cm was obtained in the conductivitytest carried out according to Example 2.

EXAMPLE 4 in a pulsating column with perforated bases having aneffective height of 3,000 mm and a diameter of 150 mm and com taining 30perforated bases with a perforation diameter of 2 mm and a free surfaceof 22 percent, the polycarbonate solution was extracted in counterflowwith water at a ratio by volume of 2:1. With a loading capacity of m /m/h, the ratio by volume of polymer solution to water in the column wasvaried by varying the stroke frequency while the lifting height was 3mm.

a. 500 strokes per minute correspond to 0.2 part of polycarbonatesolution in the washing liquid b. 300 strokes per minute correspond to0.12 part of polycarbonate solution in the washing liquid 100 strokesper minute correspond to 0.05 part of polycarbonate solution in thewashing liquid d. 30 strokes per minute.

The extraction of the polycarbonate solution according to Example 40with 60 ppm of CI in the polycarbonate and Example 4b with lO ppm of CIin the polycarbonate was unsatisfactory, whereas the experimentaccording to Example 40 yielded a perfectly extracted polycarbonate 2ppm Cl, conductivity test according to Example 2 0.1 '10 S/cm) with goodtechnological properties. Experiment 4d remained without success. Thepolymer solution accumulated on the perforated bases.

We claim:

1. A process for the purification of polycarbonate polymer solutions bywashing them with a detergent solution in pulsating columns havingperforated bases, characterized in that a proportion by volume of thepolymer solution to the detergent solution s 0.1 is maintained in theextraction column and the detergent solution is used as the continuousphase.

2. A process according to claim 1, characterized in that the proportionby volume of the polymer solution in the detergent solution is regulatedby the following means. alone or in combination:

a. by varying the distance between the perforated bases,

b. by varying the free surface of the perforated bases,

c. by varying the pulsation frequency and lifting height,

d. by varying the difference in density between the polymer solution andthe detergent solution.

3. A process according to claim 1, wherein the polycarbonate solution iswashed with dilute alkali solution, dilute acid and water.

4. A process according to claim 1, characterized in that the viscositiesof the polymer solution are between 10 and cP.

5. A process according to claim 1, characterized in that a pulsatingcolumn with perforated bases in which the distance between theperforated bases is from 5 to l5 cm and the perforated bases have atotal open area of from 15 to 30 percent and a perforation diameter offrom 1.5 to 4.0 mm, a lifting stroke frequency of from 50 to strokes perminute and a lifting height of from 2 to 5 mm is used with a differencein density between the polymer solution and the detergent solution of0.l g/cm and a proportion by volume of the polymer solution to thedetergent solution within the limits of from 10:1 to l:l under a load offrom 10 to 20 m /m lh.

2. A process according to claim 1, characterized in that the proportionby volume of the polymer solution in the detergent solution is regulatedby the following means, alone or in combination: a. by varying thedistance between the perforated bases, b. by varying the free surface ofthe perforated bases, c. by varying the pulsation frequency and liftingheight, d. by varying the difference in density bEtween the polymersolution and the detergent solution.
 3. A process according to claim 1,wherein the polycarbonate solution is washed with dilute alkalisolution, dilute acid and water.
 4. A process according to claim 1,characterized in that the viscosities of the polymer solution arebetween 10 and 100 cP.
 5. A process according to claim 1, characterizedin that a pulsating column with perforated bases in which the distancebetween the perforated bases is from 5 to 15 cm and the perforated baseshave a total open area of from 15 to 30 percent and a perforationdiameter of from 1.5 to 4.0 mm, a lifting stroke frequency of from 50 to150 strokes per minute and a lifting height of from 2 to 5 mm is usedwith a difference in density between the polymer solution and thedetergent solution of >0.1 g/cm3 and a proportion by volume of thepolymer solution to the detergent solution within the limits of from10:1 to 1:1 under a load of from 10 to 20 m3/m2/h.